The production of refined primary steel is usually based upon the refining of molten impure iron, known as hot metal, with oxygen. Typically, one or more jets of gaseous, commercially pure oxygen are impinged upon or blown through the hot metal to oxidize its dissolved impurities. The hot metal is usually produced in a blast furnace by the smelting of iron ores with carbon; or less commonly in a cupola or other melting furnace by the melting of pig iron and scrap. The refining operation is organized in successive batches, called heats.
Refining furnaces for primary steel may be of several configurations, the most common being the upright basic oxygen furnace. There are two principal varieties of the upright basic oxygen furnace: One variety is top-blown through a movable oxygen lance, and the other, known as the Q-BOP, is bottom-blown through submerged oxygen tuyeres. The inclined rotary basic oxygen furnace and the basic open-hearth furnace are less common primary steel refining furnaces. The electric-arc furnace process is normally used to melt and refine secondary steel from scrap, but in some instances may be charged in part with hot metal, or with pre-reduced iron ore, and used as a primary steelmaking furnace.
The impurities commonly found in smelted hot metal from a blast furnace (or in hot metal melted from scrap and pig iron) are carbon, silicon, manganese, sulfur, and phosphorus. Lesser impurities could include aluminum, titanium, and any of the heavy metals, plus dissolved gases such as nitrogen. At high temperatures, and in the presence of a supply of oxygen, the elements aluminum, titanium, silicon, carbon, manganese, and phosphorus may be oxidized appreciably. The oxides thus formed are insoluble in molten iron. Being less dense than the molten iron, such oxides will rise to the top surface of the hot metal as it is being refined. The oxides of carbon, being gaseous, promptly escape into the furnace atmosphere and the exhaust system. Most of the other oxides produced by refining will float as a dross on the surface of the metal. If these oxides are suitably fluxed, they will form a slag that is molten at practical steelmaking temperatures.
The refining of hot metal with oxygen causes the oxidation of a significant portion of the iron together with its impurities. At the conclusion of the refining process, the slag will therefore contain both ferrous and ferric oxides. In general, the concentration of iron oxides in a steel refining slag tends to become higher as the carbon content in the refined primary steel becomes lower.
The sulfur content of the hot metal is initially controlled during smelting to relatively low concentrations. This sulfur may be in part shifted to the refining slag during steelmaking. Slagging and removal of sulfur is facilitated if the slag is constituted (by flux additions) to include a preponderance of lime and/or magnesia such that it is basic in character. (The term basic, as applied to steelmaking slags, denotes an excess of basic oxides, such as lime and magnesia, relative to so-designated acid oxides such as silica and alumina). A strongly basic slag may also assist in the oxidation and removal of phosphorus. Finally, a basic slag helps to limit erosive attack by the slag upon the lining of the refining furnace. The lining consists principally of carbon-bonded periclase or magnesia brick, plus rammed materials made from periclase or other magnesium oxides.
A conventional basic steelmaking practice embodies the use of dead-burned lime as a flux charge. The lime charge is calculated so that the refining slag will attain a final lime-to-silica ratio in the neighborhood of 3:1. Dolomitic (magnesia-bearing) lime is sometimes used to increase the magnesia content of the slag and thereby further limit the erosion of the furnace lining by solution of its magnesia. Fluorspar, calcium fluoride, may be added to the furnace to help a fluid slag to form early in the refining operation.
The final products of oxygen refining as described above are molten refined steel at high temperature, and a covering layer of fluid slag. The refining processes are mainly exothermic, and require the use of scrap steel and/or iron ore in the furnace charge as coolant. This is advantageous in the context of primary steel production, in that modern oxygen steelmaking processes can absorb and recover practically all of the scrap generated during processing of raw primary steel into saleable shapes.
A typical charge to a basic oxygen steelmaking furnace is 65 to 75 percent hot metal and 25 to 35 percent steel scrap. Fluxes are added as required to develop the desired refining slag, and some iron ore may be added late in the heat as a "trimming" coolant. The scrap charge is sometimes preheated to permit its use in larger proportions.
The final temperature of the refined steel is normally 2900.degree. to 3000.degree. F. (1593.degree. to 1649.degree. C). The molten refined steel is generally tapped from beneath the molten slag layer into a refractory-lined receiving ladle. During the tapping of the heat, it is common to add deoxidizing and alloying materials to the receiving ladle. The ladle of refined steel may be taken to a pouring station, or to a separate site for additional treatments such as degassing. Eventually, the molten steel is teemed into ingot molds, or into a continuous casting apparatus.
After the steel has been tapped, the refining slag is poured out of the refining furnace, usually into a slag pot made of cast steel. This slag discharge may also include any molten steel remaining in the furnace, notably as droplets of metal entrained in the slag. The slag is taken to a slag processing area, where it is poured out, cooled, and broken. Such slag is usually processed through a magnetic system to reclaim steel droplets and spillage metal for recycling. The processed slag, after removal of its contained free metal, may be used as a charge to the blast-furnace (smelting) process, which retrieves the iron and manganese values together with useful flux values.
It will be apparent from the foregoing description that the refining slag is regarded as a process waste. Such slag is presently useful only as a source for the secondary and separate recovery of contained metal, by means of magnetic processing and/or re-smelting. All such secondary recovery work is performed after the slag has first been cooled and solidified, at a considerable loss of sensible heat and chemical reaction potential. Yet is is well known that the refining slag is rich in iron oxides and other active oxidizing agents. And it is similarly well known that great advantages could accrue if this liquid slag could be made to react directly with the hot metal as it arrives in the steelmaking plant ready to be refined.
As stated above, the potential benefits of a reaction between fresh hot metal and steel refining slag are well known. To be specific, the reaction was of interest during the early evolution of steelmaking, especially the open-hearth, Bessemer (air pneumatic), and combined ("duplex") processes. Development of these processes was marked by several proposed techniques based upon retaining (in the refining furnace) all or part of the liquid slag from the previous heat. The oxidizing power of the slag was used to partly refine the incoming hot metal, and at the same time the carbon and silicon in the hot metal acted to reduce and recover the contained iron and manganese in the slag.
This principle is recognized in the early patents of Benjamin Talbot, such as U.S. Pat. Nos. 599,290; 688,557; 747,661 and 747,662. Other similar contributions to the art are described in U.S. Pat. Nos. 694,752; 788,650; 898,513; 927,097; 1,137,681; 1,198,827; 1,254,078; 2,111,893; 3,254,987; and Austrian Pat. No. 327,255. It is to be understood that the above list of patents is not intended to be exhaustive; however, it illustrates the origin and development of this aspect of the art of steelmaking.
Rene Perrin, commencing in about 1935, further advanced the general art of slag/metal reaction by developing controlled techniques, including manipulative methods, for bringing molten oxidizing slags together with molten metals containing reducing agents. Perrin's work included pouring metals through slags, pouring metals and slags together from one ladle to another, and simultaneously pouring slag and metal from two ladles into one. His processes are exemplified by present methods for making low-carbon ferrochromium from the reaction between molten chromium silicide and a slag made from chrome ore and lime. These methods and techniques are described in U.S. Pat. Nos. 2,015,691; 2,015,692; 2,123,658; 2,310,865; 2,767,077; 2,767,078; and 2,767,079. In addition, Jean Rouanet obtained U.S. Pat. Nos. 3,617,257 and 3,702,695 for more recent developments in this area. Again, it is to be understood that this list of patents is representative of the development of controlled slag/metal reactions, and is not exhaustive.
Perhaps the closest approach to a practice that could properly utilize the reactive, thermal, and material character of steel refining slag, in modern oxygen steelmaking operations, is described by Ernest Glaesener in U.S. Pat. No. 3,151,976. Glaesener advocated retaining the refining slag in the steelmaking furnace so that it could be reacted with the next incoming charge of fresh hot metal. But, as Glaesener observed, the reaction is explosively rapid under the usual conditions of hot metal charging. Modern basic oxygen steelmaking depends upon a rapid operating pace. Haste is imperative, both for productivity and for prudent thermal and chemical process control. The emphasis on pace usually means that hot metal is poured into the refining furnace as fast as is physically possible; the pouring rates approximate or exceed 100 tons per minute. At this pace of mixing, the reactions between hot metal and refining slag necessarily become violent. This is because the reactions release considerable gas as the reaction product between carbon from the hot metal and oxides from the slag.
Glaesener correctly noted that a slow-paced initial pouring of the hot metal charge would alleviate the explosive tendency, but at a cost in lost time. To solve the problem, he advocated the technique of charging modest amounts of solid, granular reducing agents (such as carbon, or granular pig iron) into the slag ahead of the hot metal. Such reducing agents would pre-react with the slag and somewhat alleviate its initial potential for violent reaction.
The foregoing description of the prior art amply establishes concepts important to an understanding of the present invention. First, it is established by the work of Talbot and others that the expected effects of an induced reaction between hot metal and steel refining slag are known in the practical sense, and are known to be beneficial. Second, it is established by the work of Perrin and others, that the control and management of a reaction between metal and slag is attainable. The applicable arts include pouring metal into and through slag, pouring metal and slag together into a receiving vessel, and/or pouring metal and slag together between two containers. Third, it is established by the work of Glaesener that the implementation of a slag/metal reaction, in the context of modern high-speed basic oxygen steelmaking, is in and of itself a problem in practical operating metallurgy. The development of techniques that circumvent or nullify the basic conflict between the need for close control (on the one hand) and the need for fast overall pace (on the other) is to be desired.
Thus, several steelmakers have tried, on occasion, pouring fresh hot metal into retained refining slag. It was found, however, that the resulting reaction is too violent and dangerous at the usual rapid pace of hot metal charging operations. It was also found that the slow-down of the hot metal charging operations, to compensate for the exothermic character of the slag reaction, results in intolerable losses of time. In summary, none of the workers in this area ever found a way to incorporate the slag/hot metal reaction practically and safely into modern steelmaking practice.